The discovery of RNA ligase enzymes in DNA phage infected bacteria as well as mammalian cells opens up two new areas for investigation. First, the biological role of enzymes capable of covalently joining RNA chains is potentially significant per se, including as possibilities 1) host RNA modification by infecting viruses, 2) post-transcriptional alterations of various RNA classes in normal prokaryotic or eucaryotic cells and 3) recombination and/or repair of RNA viruses. The second aspect represents essentially systematic exploitation of these enzymes to create defined RNA oligomers or higher analogues of native species which should have major biological as well as biophysical applications. Among these can be mentioned site specific sequence alteration of molecules such as tRNA's, rRNA's and viral RNA's, and the preparation of novel RNA molecules for investigating such physicochemical properties as the stability of hair-pins, bulges or other configurations.